1. Field of the Invention
The present invention relates to a spectral purity filter for a multi-layer mirror, a lithographic apparatus including such multi-layer mirror, a method for enlarging the ratio of desired radiation and undesired radiation and a device manufacturing method wherein such a multi-layer mirror is used.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation. Such radiation is termed Extreme Ultra Violet (EUV) or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
The source of EUV radiation is typically a plasma source, for example a laser-produced plasma or a discharge source. In some instances, an EUV source may emit several different wavelengths of light, some of which may be undesired radiation, such as deep ultra-violet radiation (DUV). This non-EUV radiation can, in some instances, be harmful for the EUV lithography system because it can result in a loss of contrast. Thus, it may be desirable to have such undesirable radiation removed by a spectral purity filter (SPF).
Present spectral purity filters are based on blazed gratings. These gratings are often difficult to produce, since the surface quality of the triangular shaped pattern should be high. For example, for some gratings the roughness of the surface should be lower than 1 nm RMS. In addition to that, the current spectral purity filter folds the path of the light, which means that it cannot be removed from the system unless another mirror is used to replace the spectral purity filter, thereby causing losses. In order to keep flexibility it may be desirable to be able to remove the spectral purity filter from the system at will, in order to evade the losses caused by the spectral purity filter.